The electromechanical element has a wide variety of application fields such as radio field, optical field, acceleration sensor, biotechnology, and others. Out of them, this device is applicable to components such as a switch, a filter, and the like for a radio equipment.
Through the spread of information communication devices such as a radio terminal, and the like, a broader band of a frequency used in communication is accelerated from several hundred MH band for a cellular phone, and the like to several GHz band for a wireless LAN, and the like. In the present situation, the terminals that comfort to various communication protocols are employed independently. In the future, the realization of a small-size terminal that can comfort to various communication protocols as one radio terminal is desired. In such a tendency that an increase of the number of passive components such as switches, and the like built in a case of the terminal, a size reduction of the passive components is requested.
In such circumstances, a research and development of a high-frequency electric machine (RF-MEMS: Radio Frequency MEMS) manufactured by the MEMS (Micro Electro Mechanical Systems) technology is stimulated. The “electromechanical switch” denotes a switch that switches a signal propagation path mechanically by moving a micro movable electrode. Its advantage is that high frequency characteristics such as ultra low loss, high isolation, and the like are excellent. Also, since this electromechanical switch can be manufactured by the process that has a good affinity for a RF-IC, such switch can be built in the RF-IC. Therefore, the electromechanical switch is expected as the technology that can contribute largely a downsizing of the radio portion.
As the electromechanical switch in the prior art, the switch set forth in Patent Literature 1 has been known. The electromechanical switch has a membrane-like or a rod-like movable electrode as a simple beam structure or a cantilever structure, and switches a signal propagation path by causing the movable electrode to connect/disconnect to/from a fixed electrode. Most of the electromechanical switches employ an electrostatic force as a driving power source of the membrane or the moving body.
At this point in time, following problems exist in the utilization of the electromechanical switch in radio communication.
In order to acquire the quick response characteristic, a high driving voltage is needed. In order to move a movable electrode that has a finite mass while using an electrostatic force as a driving force, a strong electrostatic force, i.e., a high driving voltage is needed.
That is, when improvement of a response speed is intended, a control voltage for driving the movable electrode must be set to an extremely high potential. In LSI in which a lower power supply voltage is advancing, it is difficult to satisfy such request.
For example, the switch using the semiconductor device in the prior art can get a quick response in order of nanosecond (ns). But the electromechanical switch can get a response merely in order of about several tens μs, and a response speed is very slow.
Most of micro electromechanical switches are of the type whose movable electrode is driven by an external force and then is restored (released) into an original position only by a spring force that the movable electrode itself possesses. Therefore, such a problem existed that, when a spring force is weakened to realize the quick response/low voltage drive, a release speed becomes slow.
In order to solve this problem, as set forth in Patent Literature 1, such an approach can be considered that a spring constant of the movable electrode should be increased by providing a convex structure on the movable electrode.
FIGS. 12A and 12B are views showing an electromechanical switch in the prior art, wherein FIG. 12A is a sectional view showing an OFF state, and FIG. 12B is a perspective view showing an ON state. An electromechanical switch 100 has a configuration of the series type (series-connected type) switch that, when a movable electrode 101 comes in contact with a lower electrode 102, a high-frequency signal is propagated to the output side and, when a contact of the movable electrode 101 to the lower electrode 102 is cut off, a high-frequency signal is cut off. When a configuration as the electromechanical switch shown in FIG. 12A is viewed, the lower electrode 102 on a surface of which an insulating film 103 is formed and the movable electrode 101 that are bridged between posts 104 as the simple beam are provided on a substrate 105 on a surface of which an insulating film is formed. When the electromechanical switch 100 is turned ON, a voltage VON is applied between the movable electrode 101 and the lower electrode 102, as shown in FIG. 12B, and the electromechanical switch 100 is driven toward the lower electrode 102 side by an electrostatic force. In this case, the movable electrode 101 comes in contact with the lower electrode 102, and a propagation path of the high-frequency signal is formed by a capacitive coupling via the insulating film 103. In contrast, when the electromechanical switch 100 is turned OFF, the voltage VON is cut off, as shown in FIG. 12A, and the movable electrode 101 and the lower electrode 102 are set to the same potential. In this case, the movable electrode 101 is driven upward by a spring force that the movable electrode 101 itself possesses and is restored into its original position. Thus, the signal propagation path between the movable electrode 101 and the lower electrode 102 is disconnected.
That is, in this structure, as a first stage, the movable electrode is driven until convex structures come into contact with a lower surface of the movable electrode. In this while, a portion of the movable electrode, which is bridged between the posts, gives a spring constant. Then, as a second stage, a portion of the movable electrode, which is bridged between the convex structures, is driven downward and is brought into contact with the lower electrode. In this while, a portion of the movable electrode, which is bridged between the convex structures, gives a spring constant. Since a length of the portion that is bridged across the hollow portion can be changed in respective stages, a spring constant can be changed and increased. In this case, an increase of a release speed produced by increasing a spring force can be expected.
Patent Literature 1: WO02/96796